In a fuel cell system which uses hydrocarbon as a raw material gas, a reforming reaction of the raw material gas is executed. To accelerate the reforming reaction, a reforming catalyst is used. However, since the raw material gas contains, for example, a sulfur compound as an odorant, the reforming catalyst may deteriorate by the sulfur compound. To suppress the deterioration of the reforming catalyst, a desulfurizer which removes the sulfur compound in the raw material gas is utilized.
One example of such a desulfurizer is a hydro-desulfurizer which performs hydrodesulfurization of causing the sulfur compound of the raw material gas to react with hydrogen on a catalyst to convert the sulfur compound into hydrogen sulfide and removing this hydrogen sulfide.
The hydro-desulfurizer is heated to a temperature suitable for a hydrogenation reaction and requires the hydrogen when performing the desulfurization. In many cases, the fuel cell system uses a method of: adding a part of a hydrogen-containing reformed gas generated by the reformer to the raw material gas before the raw material gas flows into the hydro-desulfurizer; and performing the hydrodesulfurization.
Generally, the reformed gas has a high temperature of not lower than 500° C. Therefore, if such a reformed gas is added to the raw material gas, auxiliary devices and the like constituting the fuel cell system may be damaged. Therefore, according to PTL 1, as shown in FIG. 8, a recycled gas supply passage 113 is provided, through which a part of the reformed gas from a reformer 104 flows into a raw fuel supply passage 110. With this, the recycled gas is cooled down by heat radiation to the atmosphere, and the hydrogen required by the hydro-desulfurizer is supplied to the hydro-desulfurizer. Further, according to PTL 2, as shown in FIG. 9, a heat exchanger which performs heat exchange between the recycled gas and the reforming water is disposed on a circulation passage R4 serving as the recycled gas supply passage. With this, the recycled gas is cooled down, and the heat of the recycled gas is recovered by the reforming water. Thus, the efficiency of the fuel cell system is increased.